This invention relates to a semiconductor laser device which is operable with low noise even in high ambient temperature.
Recently, a semiconductor laser device is widely used for a light source in the field of a optical disk unit and an optical communication system. In particular, it is required that the semiconductor laser device used for the optical disk unit has a low optical feedback noise on readout of data from an optical disk. On such a requirement, it is well known that a self-pulsation laser device generates laser light which has a low coherence characteristic and the low optical feedback noise. Under the circumstances, if the self-pulsation laser device is used for the light source of the optical disk unit, it is unnecessary to use a high frequency module for suppressing the optical feedback noise and is no need a countermeasure against electromagnetic radiation. As a result, it is possible to provide the optical disk unit with a reduced size and a reduced cost.
In the meantime, self-pulsation operation can be obtained by forming a saturable absorbing region within a laser resonator and by controlling a saturable absorbing quantity. Such a self-pulsation operation and a laser structure are disclosed in "(Extended Abstract of 18th Conference on Solid State Devices and Materials), page 153, a paper number of D-1-2, page 21 of Proceedings in 11-th semiconductor laser symposium". Furthermore, six examples of the self-pulsation laser are reported in Extended Abstracts (41st spring meeting, 1994), the Japan Society of Applied Physics and Related Societies. The report discloses a self-pulsation laser structure that a part of an active layer positioned at both sides of a ridge waveguide is utilized as the saturable absorbing region.
The above-mentioned semiconductor laser device has, however, the following disadvantage. Namely, a conventional semiconductor laser device stops the self-pulsation operation in high ambient temperature. On the other hand, it is anticipated that the light source of the optical disk unit is used in the ambient temperature of 60.degree. C. at most. In this case, the above-mentioned semiconductor laser device is unsuitable for the light source of the optical disk unit which may be used in a high ambient temperature because the noise of the laser device increases.